Sensors for carbon monoxide are required, for example for monitoring heating installations on the basis of fossil fuels or of internal combustion engines. In both cases, the objective is, both for economical and ecological reasons, to burn the fossil fuels or the engine fuel as completely as possible. The combustion air is therefore used in a certain excess, so that the exhaust gas still contains oxygen but nevertheless in addition contains small amounts of carbon monoxide. In order to optimize the combustion processes it is useful to know the content of carbon monoxide in the exhaust gas.
Sensors for determining the carbon monoxide content in exhaust gases on the basis of metal oxides whose electric conductivity changes at the prevailing high temperatures as a function of the carbon monoxide content but also of the oxygen content, are known. Thus, the German Patent 26 03 785 describes sensors made of chromium(III) oxide or tin(IV) oxide, which are doped with at least one oxide of the transition metals of the 4th to 6th group of the Periodic Table of the Elements PTE or an oxide of iron, nickel, cobalt, tin, magnesium, calcium or lithium.
The German Patent 26 48 373 discloses sensors comprising semiconductors, which consist of doped tin (IV) oxide, the catalytic activity of the semiconductor for the reaction between oxygen and oxidizable components such as carbon monoxide being specifically reduced. Sensors on the basis of cerium(IV) oxide which is doped with magnesium oxide, aluminum oxide, yttrium oxide, titanium(IV) oxide, tantalum(V) oxide, niobium(V) oxide or vanadium(V) oxide are disclosed in the German Patent 30 24 449.
Finally, in "Sensors and Actuators", 6 (1984), 35-50, there is an article on the influence of the catalytic activity of semiconducting metal oxides on their properties as sensors for carbon monoxide, together with an exhaustive overview of the relevant literature.
Sensors of the type described have become established in practice and on the whole have useful. However, they leave something to be desired where it is a matter determining small amounts of carbon monoxide in gas mixtures whose temperatures fluctuate. In that case, the sensors must combine a high sensitivity with a Low resistance-temperature coefficient. In other words, on the one hand even small fluctuations in the carbon monoxide content should result in distinct changes in the conductivity or the resistance, and on the other hand, the conductivity or the resistance should be, as far as possible, largely independent of the measuring temperature. Furthermore, the prior art sensors are not satisfactory in terms of their useful life. It would therefore be desirable to develop sensors having longer service lives.